• wrong language code (400)
• mul is not a valid language code for search

{
  "code": "invalid-query-parameter",
  "message": "Invalid query parameter: '{query_parameter}'",
  "context": {
    "parameter": "{query_parameter}"
  }
}

• both the search query parameter (q=) and the language= query parameter are mandatory and validated via MediaWiki's REST framework

Task breakdown notes:

• create UseCaseError class for the search domain
• don't recreate ValidatingRequestDeserializer